define generic tests in configured tests-paths directory

[kwigley]

Add ability for all tests to be defined in the same subdirectory.

[jtcohen6]

Big idea here: Allow users to define test blocks inside the directories defined by test-paths.

Functionally, these test blocks work a lot like macros—in fact, a test block named my_cool_test will be converted internally into a macro named test_my_cool_test. That used to be explicit, user-exposed functionality; we've since made it a thing that happens behind the scenes.

To make this happen, I imagine we'd need to run the MacroParser over both macro_paths and test_paths:
https://github.com/dbt-labs/dbt/blob/6fc64f0d3b8a9e0c8c7566d3a22aef75d8ed8b3c/core/dbt/parser/read_files.py#L123-L125

That is, we need to parse generic tests (like all macros) before parsing other file types, in order to construct the contexts needed to parse those other resources. I definitely defer to @gshank's judgment here.

I'm not so concerned about reading the contents of test-paths twice, since we don't see many projects with tons of custom singular tests—it's not a recommended pattern, since they're really meant as one-offs—but I bet we could come up with a slicker, more performant way to avoid twice-reading in the future.